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<TITLE>The Hugs-GHC Extension Libraries: IOExts </TITLE>
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<H2><A NAME="s3">3. IOExts </A></H2>
<P>This library provides the following extensions to the IO monad:
<UL>
<LI>The operations <CODE>fixIO</CODE>, <CODE>unsafePerformIO</CODE> and <CODE>unsafeInterleaveIO</CODE>
described in </I>]
</LI>
<LI>References (aka mutable variables) and mutable arrays (but no form of
mutable byte arrays)
</LI>
<LI><CODE>performGC</CODE> triggers an immediate garbage collection
</LI>
<LI>When called, <CODE>trace</CODE> prints the string in its first argument, and then
returns the second argument as its result. The <CODE>trace</CODE> function is not
referentially transparent, and should only be used for debugging, or for
monitoring execution.
</LI>
<LI><CODE>unsafePtrEq</CODE> compares two values for pointer equality without
evaluating them. The results are not referentially transparent and
may vary significantly from one compiler to another or in the face of
semantics-preserving program changes. However, pointer equality is useful
in creating a number of referentially transparent constructs such as this
simplified memoisation function:
<BLOCKQUOTE><CODE>
<PRE>
> cache :: (a -> b) -> (a -> b)
> cache f = \x -> unsafePerformIO (check x)
> where
> ref = unsafePerformIO (newIORef (error "cache", error "cache"))
> check x = readIORef ref >>= \ (x',a) ->
> if x `unsafePtrEq` x' then
> return a
> else
> let a = f x in
> writeIORef ref (x, a) >>
> return a
</PRE>
</CODE></BLOCKQUOTE>
</LI>
</UL>
</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
module IOExts where
fixIO :: (a -> IO a) -> IO a
unsafePerformIO :: IO a -> a
unsafeInterleaveIO :: IO a -> IO a
data IORef a -- mutable variables containing values of type a
newIORef :: a -> IO (IORef a)
readIORef :: IORef a -> IO a
writeIORef :: IORef a -> a -> IO ()
instance Eq (IORef a)
data IOArray ix elt -- mutable arrays indexed by values of type ix
-- containing values of type a.
newIOArray :: Ix ix => (ix,ix) -> elt -> IO (IOArray ix elt)
boundsIOArray :: Ix ix => IOArray ix elt -> (ix, ix)
readIOArray :: Ix ix => IOArray ix elt -> ix -> IO elt
writeIOArray :: Ix ix => IOArray ix elt -> ix -> elt -> IO ()
freezeIOArray :: Ix ix => IOArray ix elt -> IO (Array ix elt)
instance Eq (IOArray ix elt)
performGC :: IO ()
trace :: String -> a -> a
unsafePtrEq :: a -> a -> Bool
</PRE>
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